1. Field of Invention
The present invention relates to brushless DC motors. More particularly, the present invention relates to an apparatus for controlling the speed of the brushless DC motor.
2. Description of Related Art
Brushless DC (BLDC) motors using permanent magnets are simple in design and rugged in structure. The BLDC motor omits the cumbersome and unreliable commutator and brush structure of the conventional DC motor. A dual-phase and 4-pole design of the BLDC motor is especially popular for low-power (less than 100 W) applications. If the performance and speed control of the BLDC motors keeps improving, the BLDC motors have potential to be prevalent in home appliances, air-conditioning, and machine tools, replacing induction motors and brush DC motors.
A 4-pole BLDC motor requires only one low-cost Hall sensor to operate the motor properly. FIG. 1 illustrates a schematic view of a typical 2-coil, 4-pole BLDC motor design. The armature (coil assembly) is mounted on stator 102, which has 4 poles, P1, P2, P3 and P4. Both coil 1 and coil 2 are wound on all four poles P1, P2, P3 and P4 in a bifilar fashion. Rotor 104 is made of four sections of permanent magnet (N1, S1, N2, S2) joined together. Hall sensor (HS) 106 is mounted on stator 102 between the poles P1 and P2 to sense the sectional position of rotor 104.
Hall sensor 106, which is slightly shifted toward pole P1, sends out a logic High signal (representing a high logic level) when it detects a positive magnetic flux coming into its sensing surface. Contrarily, when Hall sensor 106 detects a negative magnetic flux or no flux at all, it sends out a logic Low signal (representing a low logic level). It is noted that as a convention, positive magnetic fluxes leave from a north pole and return to a south pole.
Control circuit 108 comprises base drive circuits for driving power transistors Q1 and Q2. When control circuit 108 receives a logic High signal from Hall sensor 106, power transistor Q1 is turned on and power transistor Q2 is turned off, and thus the current flows through the first coil (Coil 1). The current of Coil 1 flows from Vin, sequentially through the poles P4, P3, P2, P1, and returns to the ground via power transistor Q1, creating outgoing magnetic fluxes on the surface of poles P1 and P3. These outgoing magnetic fluxes return to the surface of poles P2 and P4.
In other words, the current flowing through Coil 1 turns the poles P1 and P3 into north poles and turns the poles P2 and P4 into south poles. North pole P1 repels the section N1 and attracts the section S1 of rotor 104, causing rotor 104 to spin clockwise. South pole P2 repels the section S2 and attracts the section N1 of rotor 104, causing rotor 104 to spin clockwise as well. Likewise, poles P3 and P4 all act to coerce rotor 104 to spin clockwise. The mechanical force (shaft torque) developed will continue to spin rotor 104 clockwise until the section N1 is completely positioned under pole P2, and the section S1 is completely positioned under pole P1 and so on. By that time, Hall sensor 106 has engaged with the right edge of the section S1 and detects a negative flux, outputting a logic Low signal. Control circuit 108 turns off power transistor Q1 and turns on power transistor Q2 according to the logic Low signal, and then Coil 2 conducts current.
The current of Coil 2 now flows from Vin, sequentially through the poles P1, P2, P3, P4, and returns to the ground via power transistor Q2. The current flowing through Coil 2 is in a reverse direction from the situation when Coil 1 conducts current. At this moment, poles P1 and P3 become south poles, and poles P2 and P4 become north poles. Pole P1 repels the section S1 and attracts the section N2 of rotor 104; pole P2 repels the section N1 and attracts the section S1 of rotor 104, and so on. This keeps rotor 104 spinning in the clockwise direction.
FIG. 2 is a circuit diagram of a single-coil BLDC motor, which uses a full-bridge circuit to drive the motor. The full-bridge circuit comprises four power transistors Q1, Q2, Q3, and Q4. A stator of the motor has only one coil, which is wound on all four stator poles P1, P2, P3, and P4. A Hall sensor (HS) 206 is similarly mounted between poles P1 and P2. When Hall sensor 206 detects a positive magnetic flux, Hall sensor 206 sends out a logic High signal to control circuit 208, directing it to turn off power transistors Q2 and Q3, and then turn on power transistors Q1 and Q4 after a slight delay. This applies a current through the single coil, flowing from Vin, sequentially through power transistor Q1, and poles P4, P1, P2, P3, and returning to the ground via power transistor Q4. The current flowing in this direction turns the vertical poles (P1 and P3) into north poles, and turns the horizontal poles (P2 and P4) into south poles. The magnetic motive force developed on those four poles P1, P2, P3, and P4 coerces the rotor to spin clockwise.
When magnetic rotor section N1 revolves to be completely under pole P2, Hall sensor 206 engages with the right edge of magnetic rotor section S1 and detects a negative magnetic flux. Control circuit 208 receives a logic Low signal from Hall sensor 206 and thus turns off power transistors Q1 and Q4, and turns on power transistors Q2 and Q3. The full-bridge circuit now applies a current through the coil, flowing from Vin, sequentially through power transistor Q2, poles P3, P2, P1, P4, and returning to the ground via power transistor Q3. The current flowing in this direction turns the vertical poles (P1 and P3) into south poles, and turns the horizontal poles (P2 and P4) into north poles. This keeps the rotor spinning clockwise. Accordingly, the overall operation of this full-bridge circuit driving a single-coil motor is very similar to the two-transistor control circuit in FIG. 1 driving a 2-coil motor.
FIG. 3 is a circuit diagram of a conventional BLDC motor with speed control, which uses a linear regulator to control the input voltage of the BLDC motor. As illustrated in FIG. 3, linear regulator 312 with an adjustable output voltage is connected in between a 12V power source (Vsupply) and an input voltage (Vin) to the motor drive circuit, which comprises two power transistors, Q1 and Q2. A variable resistor VR adjusts the input voltage Vin to run the motor.
In general, the input power (Pin) is proportional to Vin*lin, where lin is the input current. On the other hand, the output power delivered to the motor load, Pout, is equal to the product of shaft torque and rotating speed as follows:Pout=torque*speed=Pin*efficiency
However, for a given Vin, as the rotating speed of the motor increases, the permanent magnetic sectors of the rotor will induce a counter electromotive force (EMF) on the stator windings. The faster the motor spins, the higher magnitude of counter EMF it produces. Therefore, both input current and input power decrease in proportion to the rotating speed of the motor. For a given Vin, the output power as well as the output torque decrease as the rotating speed of the motor increases.
FIG. 4 is a diagram of the shaft torque lines versus rotating speed at different input voltage levels of 12V, 10V, and 8V, of the motor in FIG. 3. It also illustrates a load torque curve of this motor driving a cooling fan. The cooling fan is basically a friction load, and the faster the fan spins, the higher torque it requires. The final rotating speed of the motor is determined by the point where the shaft torque line intersects with the load torque curve. As illustrated in FIG. 4, at 12V input voltage, the fan speed is to be at 1900 RPM; at 10V input voltage, the fan speed is reduced to 1300 RPM; and at 8V input voltage, the fan speed is reduced to 680 RPM. The main drawback of this conventional motor with a linear regulator is very poor efficiency in that the linear regulator tends to dissipate significant power loss.
FIG. 5 is a circuit diagram of another conventional BLDC motor with speed control, which applies a pulse-width modulation (PWM) to the base drive current. As illustrated in FIG. 5, the conduction time (Ton) of power transistor Q1 or Q2 is now reduced by an amount proportional to (1−D), where D is the duty cycle. A PWM speed control is equivalent to controlling the conduction time of the motor's driving current. For example, by setting the duty at 0.7, the input current as well as the input power are reduced by 30% from their value at full duty.
FIG. 6 is a diagram of the torque curves versus the rotating speed at different duty cycle values of 1.0, 0.7, and 0.5, of the motor in FIG. 5. The motor runs at 2050 RPM at full duty; the speed of the motor is reduced to 1620 RPM at duty cycle of 0.7; and the speed of the motor is reduced to 1150 RPM at duty cycle of 0.5.
FIG. 7 illustrates the PWM control scheme waveforms of the motor in FIG. 6. FIG. 7(A) illustrates the Hall sensor output; FIG. 7(B) is the base drive current waveform of power transistor Q1 before a PWM is applied; FIG. 7(C) is the base drive current waveform of power transistor Q2 before a PWM is applied; FIG. 7(D) illustrates the torque at full duty. FIG. 7(E) illustrates the base drive current waveform of power transistor Q1 after a PWM is applied; FIG. 7(F) illustrates the base drive current waveform of power transistor Q2 after a PWM is applied; FIG. 7(G) illustrates the torque when the PWM duty is at 0.7 (from T0 to T1), and when the PWM duty is at 0.5 (from T2 to T4).
However, the conventional motor with PWM speed control has some drawbacks. If the duty is set too short, for example, 0.5 or lower, the motor will have a low starting torque such that the motor may not start properly with a pre-applied heavy load. In a situation when the load of the motor fluctuates wildly, such as in a power drill, a sudden application of a heavy load may cause the motor to stall. As illustrated in FIG. 6, the light load torque curve (as the power drill drives into a soft wood) intersects the 50% duty line at 1150 RPM (S3). If the power drill encounters a piece of metal, the load suddenly increases to the heavy load torque curve. Because the shaft torque is smaller than what the actual load demands, the motor will quickly decelerate. But since the new load torque curve (the heavy load torque curve) does not intersect with the 50% duty line, the motor will stall if the control circuit cannot react in time.
Unfortunately, the Hall sensor will send out exactly 4 step signals (rising and falling edges) per mechanical revolution. For example, when the rotating speed is at 120 RPM, the Hall sensor only toggles 8 times per second. The dilemma is, before an impending stall, the rotating speed of the motor quickly decelerates, but the interval for the Hall sensor to regularly send out the step signal becomes longer. In extreme cases, a motor encountering a sudden load increase may stall so abruptly that the last signal from the Hall sensor for the control circuit to determine a motor stall never happens.
Moreover, if the applied PWM frequency is too low (<20 kHz), there will be audible switching noise. On the other hand, if the applying PWM frequency is too high (>20 kHz), there will be extra conduction loss (because the coil's AC resistance is significantly higher at >20 kHz) and higher core loss (because the eddy current loss due to the flux density changes at the higher frequency).